A magnetic resonance imaging apparatus adds positional information to a magnetic resonance signal emitted from a subject by applying a gradient magnetic field thereto, and reconstructs an image based on this positional information.
The magnetic resonance imaging apparatus applies a gradient magnetic field in pulse form. For this reason, an eddy current appears in conductive components arranged around the gradient coil (e.g., a heat shield for a magnetostatic magnet), and this eddy current generates a magnetic field (hereinafter, “eddy current magnetic field”). The eddy current magnetic field acts in a direction of suppressing the changes of the gradient magnetic field and deforms the waveform of the gradient magnetic field. Thus, corrections of the waveform need to be made in consideration of the influence of the eddy current magnetic field, or otherwise the image reconstructed from the magnetic resonance signals would be degraded. For this reason, eddy current correction has been employed to correct the waveform of the gradient magnetic field.
In the eddy current correction, calculation is performed by use of a predetermined eddy current correction parameter (intensity and time constant) with respect to an ideal waveform of the gradient magnetic field so that a corrected waveform can be output as the result. The gradient magnetic field power supply (hereinafter, “gradient power source”) applies a gradient magnetic field in accordance with the corrected waveform. With the eddy current magnetic field overlapping the gradient field, the waveform of the gradient magnetic field becomes closer to the ideal waveform.
Here, the eddy current magnetic field may have a long time constant or a short time constant. It has been considered that an eddy current magnetic field with a long time constant is the one that causes image degradation. Because the eddy current magnetic field with a long time constant has substantially the same influence onto all the positions in the imaging range, an eddy current correction parameter is prepared with reference to the center of the magnetic field, and the correction is performed at all the positions with this parameter. However, a suitable correction may not always be performed for all the positions of the imaging range, and distortion or some other degradation may occur in the image, for example, at a position away from the center of the magnetic field.